This invention relates to a new class of steel alloys especially useful for the manufacture of case hardened gears and other products made from case carburized steel alloys.
Currently, there are a number of high performance gear and bearing steels on the market. A number of these materials utilize primary carbides to achieve their high surface hardness and others use stage one or stage three tempered conditions with epsilon carbide or cementite strengthening. Primary carbides are formed when the carbon content exceeds the solubility limit during the solution treatment and large alloy carbides precipitate. This is the case in particular for secondary hardening steels using alloy carbide strengthening for greater thermal stability to improve properties such as scoring resistance. However, research indicates that primary carbide formation can have a detrimental impact on both bending and contact fatigue resistance. Formation of primary carbides can also make process control difficult for avoidance of undesirable carbide distributions such as networks. In addition, primary carbide formation in current gear and bearing steel can lead to a reversal in the beneficial residual compressive stresses at the surface. This is due to a reversal of the spatial distribution of the martensite start temperature due to the consumption of austenite stabilizing elements by the primary carbides. Thus, there has developed a need for case hardenable steel alloys which do not rely upon primary carbide formation, but provide secondary hardening behavior for superior thermal stability. This invention provides a new class of steel meeting this requirement, while exploiting more efficient secondary hardening behavior to allow higher surface hardness levels for even greater improvements in fatigue and wear resistance.
In applications of sliding wear the formation of primary carbides can be beneficial; however, in current gear and bearing steels this can lead to a reversal in the beneficial residual compressive stresses at the surface due to the consumption of elements promoting hardenability by the primary carbides.
Thus, there has developed a need for case hardenable steel alloys which do not rely upon primary carbide formation.